SiO is a material used for coating of optical lenses or for insulating films and the like, but it is not in very high industrial demand because other materials are currently more popular for such purposes. SiO is produced not only industrially but also during the course of reaction between molten Si and quartz crucibles in Si single crystal pulling processes or in Si production steps in which silica stone is reduced with carbon. However, many of these processes currently involve treatment as industrial waste. Meaningful use of SiO, such as inexpensive production of Si from SiO, would therefore provide a major contribution from the viewpoint of economy and recycling of resources.
Japanese Patent Publication No. 63-79717, for example, describes an attempt to produce Si from SiO, as a process in which SiO gas is generated from silica stone and metallic silicon and is reduced with carbon kept at a temperature of 1600-2400° C. U.S. Pat. No. 875,285 also describes a process of reducing SiO with carbon. However, because reduction with carbon results in contamination of the Si with large amounts of unreacted carbon, the Si obtained is of low purity and therefore even inexpensive SiO starting materials yield Si with low economic value.
U.S. Pat. No. 3,010,797 describes a process which uses hydrogen for reduction of SiO gas obtained by reacting silicon and silica and, particularly, the reduction is accomplished with hydrogen which has passed through palladium or the like, or with hydrogen in the presence of platinum. However, attempts to reduce SiO gas with hydrogen generally confront a problem in the need for large volumes of hydrogen. Example 1 of the aforementioned U.S. Pat. No. 3,010,797 indicates that a total volume of 90.5% Si was obtained from SiO, and this is six times the stoichiometric volume of hydrogen. Also, although palladium is used in Example 3 of U.S. Pat. No. 3,010,797, twenty times the stoichiometric volume of hydrogen was necessary to obtain 86.5% of the total volume of Si from SiO. Since one mole of Si is approximately 28 g and one mole of hydrogen is approximately 22.4 L at room temperature and 1 atmosphere, the reaction described in the aforementioned example prefers 134-448 L of hydrogen to obtain approximately 28 g of Si, even if 100% of the Si in the SiO is obtained. This is also specified in claim 1 of U.S. Pat. No. 3,010,797, where it is stated that a stoichiometric excess of hydrogen is necessary for reduction. Considered in industrial terms, it is difficult to inexpensively obtain Si in a process which prefers a few hundred L of hydrogen to obtain 28 g of Si.
Japanese Patent Publication No. 62-123009 describes a process for producing silicon wherein silicon tetrachloride, trichlorosilane, silane and a silicon alcoholate are subjected to thermal decomposition or flame thermal decomposition to produce fine granular aggregates of silicon monoxide and/or silicon dioxide, and the fine granular aggregates are reduced in a reducing atmosphere at 200° C. or above. However, silicon tetrachloride, trichlorosilane, silane and silicon alcoholates are expensive, while also having many restrictions on their handling because of their corrosive and flammable properties, and thus they have not been industrially or economically suitable.
Processes for reduction of SiO also include those making use of disproportionation reactions. U.S. Pat. No. 3,660,298 describes that SiO gas at about 1800° C. undergoes the following disproportionation reaction: 2SiO→Si+SiO2. Although U.S. Pat. No. 3,660,298 does not particularly describe the method of separating and recovering the Si produced by this disproportionation reaction, as Si and SiO2 are both liquid at 1800° C. and should precipitate from the gas in the form of a mixture, separation and recovery of the Si would not be a simple matter. Consequently, processes for obtaining Si by disproportionation reaction from SiO gas are neither industrially nor economically feasible.
An economical process for production of Si from SiO has previously been disclosed by the present inventors in Japanese Patent Application No. 2000-526444. Preferably, it is a process for the production of Si wherein solid SiO is heated at between 1000° C. and 1750° C. for decomposition to liquid or solid Si and solid SiO2, and the produced Si is separated from the SiO2. According to this process, the Si particles generated, especially above the Si melting point of 1412° C., naturally coalesce as they accumulate and thereby naturally separate from the SiO2 by-product, thus facilitating recovery of the produced Si. However, the process prefers that the SiO starting material be packed as uniformly and densely as possible into the reactor, and problems arise when the packing density is low or the packing is non-uniform to result in incomplete separation of the Si and poor Si recovery efficiency. Presumably, the reason is that the Si which is produced separates because of its low wettability with the SiO2 by-product, but when gaps are present due to incomplete packing of the SiO starting material, the produced Si collects in those areas and thereby reduces the Si recovery efficiency.
Nevertheless, SiO generated in an Si single crystal pulling processes or in Si production steps in which silica stone is reduced with carbon as described above consists of fine particles with a mean particle size of 1 μm or smaller, flakes with lengths of 1 cm or greater or amorphous masses with lengths of 1 cm or greater, and in order to achieve uniform, highly dense packing of the SiO into the reactor it is necessary to carry out granulation in the case of fine particles, or pulverization and particle size classification in the case of flakes or masses. Due to the need for such complicated steps, production of Si using the aforementioned SiO by-product as starting material has been problematic from an industrial and economical standpoint.
One of the objects of the present invention, which has been accomplished with the aim of solving the problems described above, to successfully achieve efficient separation and extraction of Si by disproportionation reaction in its produced form, without steps such as Si shape ordering or classification, even for SiO in forms which do not easily allow uniform, high-density packing, i.e. SiO in the form of fine particles, scales or amorphous masses, or mixtures thereof.